In addition to SNEP's evaluations of the entire mountain range, more detailed case studies were performed to allow key issues to be studied at the appropriate scale. The Camp Creek / Clear Creek case study was one of these efforts in which several modeling studies (see Susan Ustin's description of the ecohydrologic model) were performed to assess the relative hydrologic effect of forest management as compared to residential development.
Camp Creek encompasses 8,200 ha (20,800 ac), ranges in elevation from 980 to 2,320 m (3,200 to 7,600 ft), and is managed by the Eldorado National Forest to provide timber and other products. Clear Creek encompasses 3,100 ha (7,600 ac), ranges in elevation from 500 to 1,250 m (1,700 to 4,100 ft), and has been extensively developed with low-density housing as well as ranching. The goal of one of the modeling projects was to quantify changes in runoff timing and volume stemming from changes in land management over time. Hydrologic effects were quantified using the U.S. Geological Survey's Modular Modeling System for representative climatic conditions occurring in high-, medium-, and low-magnitude water years. Aerial photographs and a geographic information system were used to assess changes in cover density, impervious area, and other information for 1940, 1952, 1976, 1986, and 1991.
The disturbance in the logged basin during the 50-year period was primarily due to the construction of logging roads. April runoff peaks increased about 18% for the medium- and high-magnitude water years. Associated with decreasing forest cover and an increased road network, annual snowmelt and subsurface flows increased over time, and annual ground water production and evapotranspiration decreased.
Residential and road construction caused small peak-flow increases in Clear Creek over the 50-year period, but because the basin is at a lower elevation than Camp Creek, the increases were in February, March, and April. The predicted change in total runoff was due to a large increase in the surface runoff contribution to total flow. Runoff response to storms occurred faster in Clear Creek by the end of the analysis period, a result associated with changes in land condition.
The second part of the hydrologic case study focused on changes in sediment production at these two catchments during the 50-year analysis period. A U.S. Forest Service tool, the Water Yield and Sediment Model, was used to estimate sediment production from the two types of land development. In Camp Creek, peak sediment production from roads was estimated to be 4 mT/ha-yr (1.8 T/ac-yr), and over 90% of the sediment was attributed to roads. The remainder was from logging, fire, and other uses.
In Clear Creek, peak sediment production from roads was 2.6 mT/ha-yr (1.2 T/ac-yr). Fire and impervious areas associated with housing contributed 16% of the basin's total, and the remainder was attributed to roads. Predicted mean annual erosion rates due to both disturbance regimes ranged from 29 to 119 times the assumed natural rate of erosion during the analysis period.
A second normalization procedure was developed to incorporate the finding that residential construction entails more road building per area of development than logging. Under this procedure, the residential rate of sediment production was 11 mT/ha-yr (5 T/ac-yr), just over twice the rate of production from roads associated with logging. This modelling study indicated that timber harvesting and residential development have different relative scales of impact, depending on whether sediment production is normalized by road area or land area.
Related SNEP chapters:
McGurk, B. J. and M. L. Davis. Camp and Clear Creeks, El Dorado County: Chronology and hydrologic effects of land-use change. Vol. 2, chapter 52, pp. 1369-1406.
McGurk, B. J., N. H. Berg, and M. L. Davis. Camp and Clear Creeks, El Dorado County: Predicted sediment production from forest management and residential development. Vol. 2, chapter 53, pp. 1407-1420.
